Armatures are available for most commercial and household motors.
Because of cost, rewinding is not an option.
The most common armatures available are for
 LAMB electric (USA).
In our opinion, GS (general signal), Domel, and many "off shore" brands should not be rebuilt.
The cost of the armature, carbon brushes and
 bearings to rebuild a motor will generally run about 1/2 the cost of a new motor. Adding labour to that may make it un-economical.  
Please use the number stamped on the body of the motor when seeking advice. It starts with either 116 or 117 and 3 more numbers  e.g. (116-765) .
The date code is also stamped on the horn or body. It is usually 6 digits and a letter. The last 2 digits are the year (****89G)  thus 1989.

Always replace both ball bearings.
They cost about $8.00-$14.00 each.
Carbons may be re-used if they have 75% left.
They cost about $12.00-$24.00 pr. 
Do not use AFTER MARKET carbons!!